FIG. 1 is a circuit diagram showing an example of a conventional multiplier which uses a MOS transistor and is known as Gilbert Mixer. The Gilbert Mixer has a characteristic that it has a great input dynamic range and a great output dynamic range. Referring to FIG. 1, reference numeral 101 denotes a voltage source, 102 a ground, 103 a first differential signal source, and 104 a second differential signal source. Further, reference numerals 105 and 106 denote each an NMOS transistor connected at the gate thereof to the first differential signal source 103, 107 and 108 denote each an NMOS transistor connected at the source thereof to the drain of the NMOS transistor 105 and at the gate thereof to the second differential signal source 104, and 109 and 110 denote each an NMOS transistor connected at the source thereof to the drain of the NMOS transistor 106 and at the gate thereof to the second differential signal source 104. Reference numeral 111 denotes a PMOS transistor connected at the drain and the gate thereof to the drain of the NMOS transistor 107 and the drain of the NMOS transistor 109, respectively, 112 a PMOS transistor connected at the gate thereof to the drain of the NMOS transistor 107 and the drain of the NMOS transistor 109, and 113 a PMOS transistor connected at the drain and the gat thereof to the drain of the NMOS transistor 108 and the drain of the NMOS transistor 110, respectively. Reference numeral 114 denotes a PMOS transistor connected at the gate thereof to the drain of the NMOS transistor 108 and the drain of the NMOS transistor 110, 115 an NMOS transistor connected at the drain thereof to the drain of the PMOS transistor 112, and 116 an NMOS transistor connected at the drain and the gate thereof to the drain of the PMOS transistor 114. Further, reference numeral 117 denotes a load resistor connected to a node between the drain of the PMOS transistor 112 and the NMOS transistor 115, and 118 a biasing voltage source.
A V-I conversion section for converting a signal voltage outputted from the first differential signal source 103 into signal current is formed from the NMOS transistor 105 and the NMOS transistor 106. A first switching section for performing switching based on a signal voltage outputted from the second differential signal source 104 is formed from the NMOS transistor 107 and the NMOS transistor 108. A second switching section for performing switching based on a signal voltage outputted from the second differential signal source 104 is formed from the NMOS transistor 109 and the NMOS transistor 110. A current mirror for turning back current obtained as the sum of drain current of the NMOS transistor 107 and drain current of the NMOS transistor 109 is formed from the PMOS transistor 111 and the PMOS transistor 112. Another current mirror for turning back current obtained as the sum of drain current of the NMOS transistor 108 and drain current of the NMOS transistor 110 is formed from the PMOS transistor 113 and the PMOS transistor 114. A further current mirror for turning back drain current of the PMOS transistor 114 is formed from the NMOS transistor 115 and the NMOS transistor 116.
Now, operation is described. The V-I conversion section converts a voltage signal applied thereto from the first differential signal source 103 and given as a first signal into a current signal. The first switching section and the second switching section switch signal current converted by the V-I conversion section based on a voltage signal applied thereto from the second differential signal source 104 and given as a second signal to obtain a multiplication output obtained in the form of a current output.
Each of the three current mirrors turns back the same output current by converting respective output current into a gate-source voltage of a MOS transistor and sharing the gate-source voltage by a MOS transistor of the same channel paired with the MOS transistor. Accordingly, by taking out difference current between signal current according to multiplication outputs and inverted signal current according to the multiplication outputs using the three current mirrors and converting the difference current into voltages by means of the load register 117, a multiplication output can be obtained in the form of a voltage output. In other words, in the Gilbert Mixer, the three current mirrors function as current-voltage converters.
Since the Gilbert Mixer given as a conventional multiplier is configured in such a manner as described above, it has a node between a PMOS transistor and an NMOS transistor, and mismatching in characteristic between the MOS transistors gives rise to a variation of a bias voltage or the like, which makes the circuit operation unstable. In order to compensate for such a variation of a bias voltage or the like described above, it is necessary to add a complicated correcting circuit to an outputting section or the like. Therefore, the Gilbert Mixer has a subject that the circuit scale becomes great and the power consumption increases. Further, since a current mirror is used in order to carry out current-voltage conversion, there is a subject that the frequency characteristic is degraded.